For many years modern machinery control has been accomplished by the use of electro-mechanical limit switches. The conventional limit switch has usually consisted of an operator head with a movable actuator that is displaced by a machine element, which in turn has displaced a mechanically operated electro-mechanical set of contacts which results in an electrical signal that can be used to control various machine elements. These limit switches have been refined over the years to the point where they have become extremely rugged with a wide variety of operator heads and actuators from which designers can choose. Actuators have included single and double rollers, fixed and adjustable levers, buttons, spring rods, cat whiskers, etc. All have been mounted in universal operator heads that fit on standard bodies containing a mechanically operated electro-mechanical set of contacts.
In more recent years solid state proximity sensors have been used for many applications formerly controlled with limit switches. Proximity sensors eliminate the need for physical contact, are solid state, computer compatible and environmentally sealed in construction and have no moving parts. Generally these new proximity sensors have taken the form of cylindrical tubes, sometimes externally threaded, with the components generally completely encapsulated therein.
Despite the advantages of proximity sensors many users continue to use the old limit switches because of the cost of conversion, redesign of machines and control hardware, replacement parts inventory and other reasons.